Mode-locking and dumping, or mode-locking and Q-switching has more commonly been accomplished in the past with a plurality of elements rather than a single element. A mode-locked/cavity dump laser using separate elements is illustrated in U.S. Pat. No. 3,995,231 issued to Richard Howard Johsnon et al. on Nov. 30, 1976. A mode-locked/Q-switched laser using separate elements is described in U.S. Pat. No. 4,019,156 issued to William D. Fountain, et al. on Apr. 19, 1977.
In the past, cavity dumping has involved the technique of inducing birefringence in an electro-optic crystal so as to divert a pulse traveling in the laser cavity out of the cavity. In this technique, a beam traversing the laser cavity is first linearly polarized with a Brewster window, for example. A pair of electrodes placed on the electro-optic crystal, which is disposed within the laser cavity, allows an electric field to be applied, which induces a birefringence in the crystal. Induced birefringence means that the crystal is made double-refracting. When linearly polarized light passes through a birefringant crystal, the transmitted beam can become elliptically polarized. If a second polarization selective element is placed in the laser cavity, it will reflect out of the cavity a fraction of the elliptically polarized beam. This causes part of the pulse to be coupled out or dumped from the laser cavity. Such an arrangement is disclosed, for example, in U.S. Pat. No. 3,508,164 to Uchida.
The same technique for dumping can be utilized for Q-switching to prevent build up in a laser cavity. Polarization rotation for Q-switching is a standard technique in which, with a polarization element within the laser cavity, light which is originally linearly polarized, when rotated 90 degrees may be prevented from propagating back and forth in the laser cavity.
In U.S. Pat. No. 3,935,543 issued to Eguchi et al. on Jan. 27, 1976, two different physical effects are utilized in a single element. One is polarization rotation and the other is a change of the refractive index of the crystalline material. These two phenomena are utilized by energizing the material with two different sets of electrodes and therefore the effects produced by the above-mentioned patent are independent.
More recently, a single electro-optic element and a single phenomenon has been used for Q-switching followed by mode-locking. This is reported in the Laser Program Annular Report-1974; UCRL-50021-74, pps. 128-132. In this article a Pockels cell is first utilized to provide Q-switching and then 100% loss modulation is used in synchronization with the pulse round trip time in the cavity. Here no cavity dumping is mentioned.